Endothelin-1 (ET-1), a peptide comprising 21 amino acids, is the strongest known vasoconstrictor. Since its discovery in 1988 by Yanagisawa et al. [27; numerical data in square brackets relate to the attached list of references], biosynthesis, mode of action and association with diseases have been comprehensively investigated and summarized in topical review articles [1, 7, 12, 17, 24]. There are three isoforms of endothelin which are coded by different genes (endothelin-1, endothelin-2, endothelin-3) of which endothelin-1 is present in the greatest concentrations and is the most effective. Endothelin-1 is synthesized in endothelium cells, in the lung, in the heart, in the kidney and in the brain. The primary translation product of the human endothelin-1 gene is a peptide comprising 212 amino acids, preproendothelin-1 (SEQ ID NO:1). In the secretion process, a short N-terminal signal sequence (amino acids 1-17) of the preproendothelin is removed by the signal peptidase. The proendothelin obtained is then processed by the protease furin on dibasic amino acid pairs to give a biologically inactive peptide comprising 38 amino acids, big endothelin (SEQ ID NO:3), from which finally the mature, biologically active endothelin-1 (SEQ ID NO:2) is formed by means of endothelin-converting enzymes (ECEs). Endothelin acts via the bond to specific receptors which are localized on muscle cells, myocytes and fibroblasts. This bond leads to efflux of calcium, activation of phospholipase C and inhibition of Na/K ATPase. In addition to the vasoconstrictive effect, endothelin also has growth-regulating properties.
In view of the detectable and presumably numerous and serious physiological effects of endothelins, in particular endothelin-1, various assays for its immunodiagnostic determination have been developed since the time of its identification and have been used for measurements of endothelin(s), in particular in human plasmas. The results of such determinations are the subject of numerous publications.
Raised plasma concentrations of endothelin-1 and big endothelin have been described for various clinical pictures [17]. These include cardiovascular diseases [1] (inter alia pulmonary hypertension [21], atherosclerosis [13], congestive heart failure [25], myocardial infarction [20]), sepsis and septic shock [11, 22, 23], cancer [2, 3, 15, 18], etc.
The immunoassays used for the measurements of endothelins in plasma samples (cf. the review in [17]) belonged in particular to the radioimmunoassay type (with marked endothelin-1 as competitor) or to the EIA/ELISA type and aimed exclusively at the determination of endothelin or the determination of an endothelin immunoreactivity. Assays of the RIA type have low specificity and also determine related peptides containing the endothelin sequence.
However, it was found that endothelin-1 (ET-1) has an extremely short residence time in the circulation and that it is removed from the circulation after only 1-2 min [6]. Since endothelin-1 in blood and plasma is considered to be stable [6], its distribution in other tissue and its rapid and high-affinity bonding to receptors are regarded as the most important reason for the short residence time. In certain tissues and body fluids, substantially higher endothelin-1 concentrations than, for example, in plasma could consequently be determined [1, 7]. In view of these circumstances, serious doubt was cast on the validity of the determination of ET-1 in plasma samples [17]. It is in fact to be assumed that the instantaneous ET-1 concentrations determinable in a plasma sample and reflecting in certain circumstances only a transition state are not important for the physiological effects of endothelin (ET-1) but that the sum of all free and bound, e.g. tissue- and receptor-bound, physiological ET-1 concentrations present in the organism are of much greater relevance.
The determination of the ET-1 precursor, of so-called big endothelin (“big ET-1”; SEQ ID NO:3), has the advantage over the determination of ET-1 that the residence time of “big ET-1” in the circulation is substantially longer than that of the ET-1 liberated therefrom. In a number of investigations, this “big endothelin” was therefore determined instead of the actual endothelin. In particular assays of the sandwich type which permit a reliable distinction of big endothelin-1 from processed ET-1 and other endothelins were used for its specific determination [4, 8, 10].
They showed that, in certain diseases, the increased ET immunoreactivities measured can be attributed to big ET.
The selective measurement of big-ET-1 represents only a gradual improvement but not an actual solution to the problem, since big endothelin too can be processed rapidly in blood circulation to give endothelin [1, 5, 9]. It therefore likewise has a relatively short biological half-life (20-25 minutes) [10], and consequently a measured value of the big endothelin determinable in the plasma likewise represents only an instantaneous plasma concentration and does not reflect the actual physiologically effective concentrations of endothelin. ET-1 formed physiologically under the conditions of a disease but already processed and bound into tissues or to receptors is not detected in plasma in the determination of big-ET-1. The total amount of physiologically active endothelin is therefore also underestimated in a measurement of big endothelin. An attempt to perform a supplementary specific measurement of the C-terminal peptide fragment of big-ET-1 (with the amino acids 74-90 of preproendothelin or the amino acids 20-38 of big endothelin) formed in the enzymatic cleavage of big-ET-1 in addition to ET-1 showed that this peptide is even less stable than ET-1 and is therefore unsuitable for measurements [10].
The prior art discloses only one commercial competitive test (N-terminal range 18-50, commercially available from Phoenix Pharmaceuticals; use for the sepsis diagnosis described in WO 00/22439) for evaluating ranges of proendothelin outside that of big endothelin. No information has been published regarding the stability and nature of the analyte to be evaluated using this assay.
It is the object of the present invention to develop an assay method which reflects the endogenous formation of big endothelin and endothelin, i.e. the total physiological concentration and hence action of endothelin, more reliably than the determinations to date of ET or big ET in plasma.
Such a method should be valid and capable of routine use and should be capable of providing reliable values for the physiological production of ET (ET-1) and/or its precursors in various pathological conditions, in particular in sepsis or other pathological conditions in which increased values for endothelin play a role.
This object is achieved, according to the invention, by determining not ET or big ET but a comparatively long-lived prepro- or proendothelin partial peptide which does not contain the ET or big ET sequences, in particular a C-terminal partial peptide which contains at least the amino acids 168-212 of pre-proET-1, in a whole blood, plasma or serum sample of a human patient, for diagnostic purposes.
The invention is based on experimental investigations by the Applicant in which said Applicant was able to show that those parts of preproendothelin which do not represent direct precursors of endothelin comprise long-lived peptides which are suitable for measuring purposes and can be measured in blood samples reliably and with a high clinical relevance.
Endothelin-1 is formed physiologically by processing of the larger precursor molecule preproendothelin (SEQ ID NO:1) or of the secreted proendothelin obtained therefrom. During such processing, further peptides form in primarily stoichiometric amounts in addition to big endothelin (and therefrom endothelin), which further peptides, however, have to date not been the subject of scientific investigations and about whose possible further processing and stability nothing has been disclosed to date. At the beginning of the investigations by the Applicant, it was hoped that it would be possible to show that at least one of the hypothetical further peptide cleavage products is present in blood samples (whole blood, plasma or serum samples) and would prove to be relatively stable and which cleavage product might therefore be suitable to serve as a measure of the physiological formation of endothelins independently of an endothelin concentration actually measurable in plasma.
The evaluation of such a cleavage product might therefore represent the method sought for the determination of the physiological endothelin concentration or production which is designated in the Claims as determination of the “formation of endothelins”. This term is used to refer to the fact that—assuming only one route of formation, namely the single known route of formation, of endothelin-1 from proendothelin—the physiological concentrations of endothelin 1 which are formed in association with the disease can correspond only to the amount of the previously processed preproendothelin or proendothelin. If the partial peptides formed in the same stoichiometric concentration in addition to big endothelin or endothelin are stable “metabolic waste products” which are neither bound to receptors nor distributed in tissues, they must be present in the circulation. Without wishing thereby necessarily to imply a certain physiological mechanism, the “determination or measurement of the formation of endothelins” can therefore also be regarded as measurement “of the secretory activity” or of the “secretory proendothelin production”.
In this Application, the peptide fragments to be determined are characterized as “long-lived”. This term means that the residence time of the peptide fragment to be determined in the circulation (in whole blood) is considerably longer than that of endothelin or of big endothelin fragments. In particular, “long-lived” means that such peptide fragments in whole blood or a plasma obtained therefrom are not subject to further rapid proteolytic cleavage and, compared with the rate of binding of endothelin to receptors and of proteolytic cleavage of cleavable fragments, are removed at a substantially slower rate from the circulation or the metabolism.
Owing to said longer stability or “long-lived character”, in the presence of such fragments the information relating to the already elapsed secretory activities is stored for a period which is suitable at least for an unproblematic measurement. If it is assumed, for example, that the endothelin precursors are liberated in a single short-term secretion, the amount of “long-lived” fragments which are measurable after a certain time corresponds to the originally secreted amount, reduced only by an amount which is linked to the physiological half-life of the peptide fragment to be measured in the circulation. If on the other hand, for example, a more or less continuous production of the endothelin precursor during the pathological process is assumed, the former physiological production of the precursor is cumulatively reflected in the measurable concentration of a peptide fragment which is long-lived in the above sense, once again reduced only by the concentration reduction of the peptide fragment which has taken place in the same period, in accordance with its physiological clearance rate. The active endothelin or its precursor big endothelin may have long been processed or removed from the circulation in the same period and, for example, may be bound to receptors and therefore no longer measurable. The longer lived a peptide fragment is or the lower its clearance rate, the smaller is the influence of the time of measurement on the correctness of the determination of the abovementioned “formation” of a biomarker, i.e. of endothelin. A concentration constant over a relatively long period means in this context that formation and clearance hold the balance. If the concentration decreases, this may indicate that the secretion of the precursor molecule (for example of proendothelin) has ceased, for example because the molecular reservoirs are exhausted, and the concentration changes to be observed are determined only by the clearance rate.
The results of the measurement of a long-lived peptide fragment without known physiological function thus provides both quantitatively and qualitatively different results from a measurement of a fairly short-lived active peptide or its likewise relatively short-lived precursor.
The investigations by the Applicant which are described in more detail below showed that the approach described above gives fruitful results in the case of the determination of the formation of endothelins.